7.7.3 Disadvantages of Creating Many Tables in the Same Database

If you have many MyISAM or
ISAM tables in a database directory, open,
close, and create operations are slow. If you execute
SELECT statements on many
different tables, there is a little overhead when the table
cache is full, because for every table that has to be opened,
another must be closed. You can reduce this overhead by
increasing the number of entries permitted in the table cache.